The copper crusher system was designed to give results in PSI, and at lower pressures, it does pretty well. When piezo measurement gear became available, it was found that at higher pressures the copper crusher system seriously underestimated PSI. Still, back in the day, copper crusher numbers were commonly reported in PSI. P O Ackley's book is full of "PSI" numbers that were produced on a copper crusher system. If you see an old standard or number, it's hard to tell whether it's from the old system or the new one, since both are reported in the same units.
The military did not use the SAAMI copper crusher system. They had their own way of doing it. Military copper crusher numbers are not the same as CUP.
Exercising a bit of Google-fu: SAAMI puts the piezo transducer in the middle of the cartridge and rests the piston on the cartridge case. CIP in Europe drills a hole in the cartridge case so that the piston is in contact with the gas, with no brass in the path. NATO EPVAT puts the transducer at the mouth of the cartridge. The US military used the SCATP method, which is similar to SAAMI.
So, there you have four different piezo methods, and at least two different copper crusher methods. Small wonder it's confusing.
Now, is the 223 loaded to the same pressure as 5.56? I have believed that they are not. However, in light of today's exercise, and the fact that brass in the pressure path reduces readings by about 5-7000 PSI, I suspect that Ramshot's 5.56 loads might be in error. It seems possible that they have used the CIP number and the SAAMI test procedure.
So the definitive answer will probably come from someone chronographing American Eagle 223 ammunition vs. American Eagle 5.56 ammunition. They are both made at Lake City, and if anyone understands military vs. SAAMI tests, it would be them.
